Current transport networks are based on a WDM (wavelength division multiplexing) physical layer, using point-to-point (pt-pt) connectivity. A WDM optical signal comprises a plurality of transmission channels, each channel carrying an information (data) signal modulated over a carrier wavelength.
The span reach, or the distance between a transmitter and the next receiver, is limited by the combined effect of attenuation and distortion experienced by the signal along the optical fiber. A solution to increase the span reach is to place optical amplifiers between the nodes. While the amplifiers significantly increase the optical power of all optical channels passing through them, they exhibit a wavelength-dependent gain profile, noise profile, and saturation characteristics. Hence, each optical channel experiences a different gain along a transmission path. The optical amplifiers also add noise to the signal, typically in the form of amplified spontaneous emission (ASE), so that the optical signal-to-noise ratio (OSNR) decreases at each amplifier site. Furthermore, the optical signals in the co-propagating channels have different initial waveform distortions and undergo different additional distortions during propagation along the transmission medium (optical fiber). As a result, the signals have different power levels, OSNRs, and degrees of distortion when they arrive at the respective receivers, if they had equal power levels at the corresponding transmitters.
As the flexibility of today's networks is delivered electronically, termination of photonic layer is necessary at each intermediate node along a route, and therefore optimization can be performed by equalizing the system span by span. There are numerous performance optimization methods applicable to traditional networks, all based on ‘equalizing’ a certain transmission parameter of the WDM signal. It has been shown that the SNR (signal-to-noise ratio) at the output of an amplified WDM system can be equalized by adjusting the input optical power for all channels. For example, U.S. Pat. No. 5,225,922 (Chraplyvy et al.), issued on Jul. 6, 1993 to AT&T Bell Laboratories, provides for measuring the output SNRs and then iteratively adjusting the input powers to achieve equal SNRs. A telemetry path between the nodes provides the measurements obtained at one node to the other.
Performance of an optical system is also defined by the BER (bit error rate) and Q factor. BER is the ratio between the number of the erroneously received bits to the total number of bits received over a period of time. U.S. Pat. No. 6,115,157 (Barnard et al.) issued to Nortel Networks Corporation on Sep. 5, 2000 discloses a method of equalizing the channels of a WDM path based on an error threshold level for each channel in the WDM signal, set in accordance with the channel rate. The transmitter power is adjusted taking into account the attenuations determined for all channels, which attenuations are calculated according to the measured BER.
As indicated above, these traditional span engineering methods are applicable to point-to-point network architectures, where all channels of a WDM signal co-propagate along the same physical medium (fiber strand) and between same source and destination nodes.
The present invention is applicable to a photonic network where each signal travels between a different source and destination node without unnecessary OEO conversions at all intermediate nodes. Thus, the conventional pt-pt based DWDM transport boundaries disappear in this architecture and are replaced by individual wavelength channels going on-ramp and off-ramp at arbitrary network nodes. Details about the architecture and operation of this photonic network are provided in co-pending patent application “Architecture for a Photonic transport Network” (Roorda et al.), Ser. No. not yet available, filed on Jun. 8, 2001 and “Architecture for an Optical Network Manager” (Emery et al.) Ser. No. not yet available, filed on ______ 2001, both assigned to the applicant. These patent applications are incorporated herein by reference.
By removing OEO conversion for the passthru channels at the switching nodes, connection set-up and control become significant physical design challenges. Traditional channel performance optimization methods do not apply to end-to-end connections that pass through many nodes without OEO conversion. Furthermore, traditional section-by-section equalization cannot be performed; connections sharing a given fiber section now have substantially different noise and distortion impairments, determined by their network traversing history.
There is a need to provide a method for engineering connections in photonic switched networks, where the channels do not have the same source and destination node.
Traditional point-to-point WDM networks perform span and path engineering based on the worst-case rules, in that the channels are aligned to the performance of the weakest channel. This clearly is not the most advantageous way of using the network resources.
There is a need to provide a method for engineering connections, which makes a better use of the available network resources than the current equalization methods.
Furthermore, traditional networks are static, in that channel allocation is fixed and any addition or removal of a channel implies extensive engineering of all channels along the affected section(s). On the other hand, the photonic switched network to which this invention applies is provided with a routing and switching mechanism that allows automatic set-up and tear-down of connections or on request. Clearly, the traditional span and path equalization methods cannot be applied in the context of dynamical reconfiguration of connections as in the above-referred photonic switched network.
There is a need to provide a method of engineering connections by switching a connection from a current path to another or changing the configuration of the current path automatically, once the network detects that the performance parameters of the current path are below preset thresholds.